Conventional technology is now established for producing nonferrous ingots using a continuous casting machine fitted with a moveable and rectilinear casting shell composed of a plurality of concatenated casting segments or bodies with a U-shaped cross-section in succession, each one with bottom and flanks but open on the sides directed toward the contiguous segments.
The segments, which form a chain, can move along a closed path that has at least one first substantially rectilinear portion at the casting and cooling region where they define a moving canal, being supported by suitable rails.
Downstream of this rectilinear portion, the path of the chain of casting bodies continues around a braked pinion that brings the chain, now slack, downward until it reaches a second pinion, arranged on the same plane as the first and motorized and hauling, which brings it back upward and to the rectilinear portion.
A metallic belt covers the aforementioned canal, defining the fourth side of the closed and continuous shell along the rectilinear portion. The belt moves at the same speed as the casting bodies of the underlying chain, and is kept in position and under tension by three rollers. The first roller is arranged above the point where the casting nozzle or crucible introduces the molten metal; the second roller, which is motorized, is arranged above the point where the solidified metal bar leaves the continuous shell; the third roller, which is the tension roller (or pulley), has its axis compulsorily parallel to those of the first two and is in a higher position with respect to these.
Means of cooling by sprays of water or water/air strike the belt and the bottom of the casting bodies, or all four sides of the moveable shell, allowing the solidification of the metal. The spray nozzles can be regulated for areas both under pressure and under flow.
One of these machines is disclosed in EP1317980 B1. Such patent shows the machine only schematically, solely in order to explain its principle of operation; in fact, in the graphic representation, the entire system for moving the rollers for transporting the belt, including the essential tension roller, is omitted.
A practical implementation of this machine, which on the world market is often called Track and Belt in order to distinguish it from other continuous shell machines such as the Properzi Wheel and Belt machine, or the Belts and Side Dams machine, also known as Hazelett, is shown in FIG. 1.
In FIG. 1, the letter A designates the supporting structure of the machine, the letter B designates the driving motor pinion and the letter C designates the resisting pinion which causes the compression of the chain D of casting bodies E along the rectilinear portion of their path in which the molten metal is cast. F designates the metallic belt, while G and H designate the supporting rollers of the belt respectively at the point of pouring the molten metal and the point of disengagement of the casting bodies from the bar in the step of solidification, and I designates the tension roller or pulley. L designates the means of moving the rollers and the tension pulley I. The letter M designates the means, constituted by rollers or by sliders, for pressing the belt against the edges of the continuous casting shell with their movement device. N designates the cooling nozzles and O designates the crucible for pouring the metal. The dotted lines show the position of the belt and of the corresponding supporting and movement elements when it is being substituted.
In industrial use, this machine has displayed some problems.
One of these problems, which derives from the offset of the various elements, is the difficulty of succeeding in obtaining a seal between the belt and the casting bodies that is such as to prevent infiltrations of liquid metal at the start of pouring, and misuse of the belt which shortens its useful life.
In fact, in a machine of the type described above, tensioning the belt as much as possible in order to keep it as rectilinear as possible, and using generic presser means to ensure the contact between the belt and the underlying casting bodies for the whole rectilinear path, as indicated in EP1317980 B1, have produced results that are not entirely satisfactory.
In fact, even a very slight non-parallelism of the axes of the three rollers results in deformations and undulations in the belt; this is combined with uncertain positioning of the presser means and hence the malfunctioning thereof (short lifetime of the belt) and possibly also interruption of the casting.
Furthermore, owing to maintenance requirements and changing the belt, which is worn down by the thermal cycles undergone in one or more work shifts, it is necessary that both the tension roller (pulley) and the other two rollers or supporting rollers be provided with a movement away from the continuous shell (canal) which, in conventional machines, is obtained by mounting the rollers on arms that rotate about pins that are machined on the supporting structure that constitutes the footing of the entire machine and which also supports the pinions: the hauling pinion and the resisting pinion of the chain of casting bodies. Such arms can be moved with hydraulic, pneumatic or electrical movement means.
In this configuration according to the known art, it has been found that it is possible for liquid metal to penetrate between the casting bodies and the belt; this phenomenon is due to the fact that it is practically impossible, with such configuration described above, to ensure the necessary precision in the parallelism of the rollers that guide the belt and the pinions that move the casting shell and also the correct positioning of the presser means.
In fact, in industrial implementations, this type of casting machine, which can reach 4 meters in height and a length that can vary from 3 to 6 meters and beyond, has considerable measurements and it is practically impossible, even by taking the greatest care, for all the reaming operations, performed on separate parts, of all the axes of all the rotating elements described above to be parallel to within tolerances that are exact enough to prevent distortion and fleeting of the belt and the ineffectiveness of the sliders or presser rollers, and consequent penetration of metal.